Liquid-crystal display devices are now used extensively owing to their excellent characteristics (low-voltage operation; small power consumption; thin displays can be constructed; displays can be used in bright light and do not cause the viewer to suffer eyestrain). However, the most commonly used TN type display system is very slow in response compared to light-emitting display systems such as the CRT type and, further, it cannot memorize the information being displayed after the applied electric field is removed (it has no memory effect). Because of these disadvantages, many limitations have been imposed on the applications of the TN type displays to a light shutter that is required to have high-speed response characteristics, a printer head, dynamic pictures that should be driven time-division-wise, such as those in TVs etc. Therefore, the TN type displays are unsuited for these applications
Recently, a display system employing a ferro-electric liquid crystal was reported. Since both high-speed response as high as 100 to 1,000 times that of the TN type liquid-crystal display system and memory effect can be obtained with the proposed display system, ferroelectric liquid crystals are expected to be a nextgeneration liquid-crystal display device and studies and developments are currently being made extensively.
The liquid-crystal phases of ferroelectric liquid crystals belong to the chiral smectic phases of the tilt type, but from the practical standpoint, those exhibiting a chiral smectic C (hereinafter referred to as Sc*) phase, which is the lowest in viscosity among the chiral smectic phases, are most preferable.
A large number of liquid-crystal compounds that exhibit an Sc phase have been synthesized and studied. For use as a ferroelectric display device, such liquid-crystal materials are required to: (A) exhibit an Sc* phase over a wide temperature range including room temperature; (B) have a proper phase sequence on the high-temperature side of the Sc* phase, with the helical pitch thereof being large, in order to obtain good orientation; (C) have proper tilt angles; (D) have low viscosities; and (E) show spontaneous polarization that is strong to some degree. However, there is no known liquid-crystal compound which alone satisfies all of these requirements.
Therefore, such a liquid-crystal compound exhibiting an Sc* phase (hereinafter referred to as an Sc* compound) is blended with other Sc* compound(s) etc. and used as a liquid-crystal composition that exhibits an Sc* phase (hereinafter referred to as an Sc* liquid-crystal composition). In any case, an Sc* compound is required to have a low viscosity and to show spontaneous polarization not lower than a certain level, particularly in order to attain high-speed response.
It is also possible to add an optically active compound as a chiral dopant to other liquid-crystal compound or composition that shows a smectic C (hereinafter referred to as Sc) phase and to use the resulting mixture as an Sc* liquid-crystal composition. Since liquid-crystal compositions obtained by this method can have low viscosities, higher-speed response is attainable and, hence, this method is commonly employed. Although the compound to be used as a chiral dopant is not necessarily required to per se exhibit an Sc* phase, compositions containing the compound should have the properties (A) to (E) as specified above with showing an Sc* phase.
Thus, the conventional Sc* compounds have insufficient performances with respect to viscosity, induced spontaneous polarization and helical pitch, etc. and, hence, have failed to provide liquid-crystal materials with good high-speed response characteristics. Therefore, there has been a desire for improvement of such conventional Sc* compounds.
It is known that in order to induce strong spontaneous polarization, the liquid-crystal compound should contain a group with a strong dipole moment at a position that is as close as possible to the core of the liquid-crystal compound molecule and also to an asymmetric carbon. Although carbonyl group is generally known as a group having a strong dipole moment, cyano group can be mentioned as a group having an even stronger dipole moment. However, known as a compound in which a cyano group is directly bonded to an asymmetric carbon are only a compound represented by the formula ##STR4## (disclosed in Proceeding for the 12th International Liquid-Crystal Conference and in JP-A-61-243055; the term "JP-A" as used herein means an "unexamined published Japanese patent application") and a compound represented by the formula ##STR5## (disclosed in U.S. Pat. No. 4,777,280 and in Walba et al., J. Org. Chem., 54, 4339(1989)), and there have been no known compounds in which a cyano group is located closer to the core.